1. Field of the Invention
The present invention relates to electrocardiographs, and more particularly to a sensitivity setting device which automatically sets an appropriate sensitivity in a device which senses a faint living body signal, and an electrocardiograph which includes such sensitivity setting device.
2. Description of the Related Art
Conventionally, an electrocardiograph used for diagnosis of heart diseases such as irregular pulses and/or myocardial infraction is known. The electrocardiograph picks up a faint voltage produced each time a myocardium shrinks from electrodes stuck with a paste, for example, on a patient's breast, amplifies the voltage, and records it as an electrocardiogram waveform (or electrocardiogram).
The maximum voltage level of the electrocardiogram waveform measured by the electrocardiograph varies from several hundreds of microvolts to several scores of millivolts depending on the patient or the manner in which the electrodes are attached to the patient. Thus, in measurement, the sensitivity (amplification factor) is required to be adjusted so as to provide an optimal dynamic range.
To this end, recently, electrocardiographs which include a sensitivity setting device which automatically sets sensitivity to provide an optimal dynamic range have been put to practical use. If the sensitivity is switched during recording of the electrocardiogram waveform, the electrocardiogram provided for diagnosing purposes becomes difficult to view. Thus, in the electrocardiograph of this type, a sensitivity determining period prior to starting the recording of the electrocardiogram waveform is provided to thereby set an optimal sensitivity for measurement of the electrocardiogram waveform during the sensitivity determining period.
However, in such conventional sensitivity setting device, the following problems arise. It is known that the electrocardiogram waveform greatly sways due to fluctuations of a reference potential level called a baseline fluctuation BD, as shown in FIG. 2B, in the initial state of the electrocardiogram investigation directly after the electrodes are set when the contact between the electrodes and the patient's skin is not electrically stable.
When the baseline fluctuation BD occurs in the sensitivity determining period where the automatic sensitivity is set, the difference between the maximum and minimum waveform values of the electrocardiogram waveform is large, and the sensitivity is conventionally automatically set to provide a dynamic range conforming to the difference. Hence the sensitivity for the electrocardiogram waveform in this situation becomes very low.
Thus, the electrocardiogram waveform conventionally recorded in the above described situation is very small and not suitable for diagnosis. Even in a digital sampling-type electrocardiograph, resolution becomes coarse in the sampling and only an, electrocardiogram waveform having a low resolution is obtained even when enlarged and displayed.
Accordingly, a sensitivity setting device has been long awaited which is capable of measuring with an optimal sensitivity a small signal superimposed on a reference voltage without being influenced by possible fluctuations of the reference voltage (baseline).